1. Field of the Invention
The present invention generally relates to an actuator arm for a disc drive apparatus. More particularly, the present invention relates to an actuator arm with a vertical coupling feature for compensating for arm bending induced track misregistration (TMR) in disc drives.
2. Description of Related Art
A hard disc drive (HDD) unit generally uses a spinning storage medium (e.g., a disc or platter) to store data. A read-write head is positioned in close proximity to the spinning storage medium by a Head Stack Assembly (HSA). Mounted on the HSA, a suspension assembly commonly includes a base plate, a load beam, and a flexure trace gimbal to which a slider is mounted. The suspension is mounted to a support arm, also called an actuator arm or a suspension arm. The slider supports the read-write transducer head element. The load beam is generally composed of an actuator mounting section, a spring region, and a rigid region. The spring region gives the suspension a spring force or preload counteracting the aerodynamic lift force created by the spinning medium during reading or writing. A gimbal is mounted at the distal end of the load beam and supports the slider allowing the head to have pitch and roll movement in order to follow the irregularities of the disc surface.
Demand generally requires increased HDD storage capacity, which generally compels higher data track densities for the storage medium. Furthermore, the demand for faster rates of data seeking and accessing also leads to higher rotational speeds. A significant obstacle associated with increasing rotational speeds and storage capacity is often head positioning accuracy as the head flies above the spinning storage medium.
A significant obstacle to head positioning accuracy is disc flutter. Disc flutter is an aero-elastic instability induced by the coupling of the spinning storage medium and the air surrounding the media resulting in disc vibration modes. These flow induced vibrations can physically cause an off-track misalignment of the head to the desired track resulting in failure to access or write data on the right track. The lateral (Y-axis) movement of the track associated with vertical (Z-axis) movement of the track due to disc flutter is characterized by:
      VC    =                                            ⁢                              Δ            ⁢                                                  ⁢                          Y              track                                +                      Δ            ⁢                                                  ⁢                          Y              head                                                  Δ        ⁢                                  ⁢        Z              where                    Δ        ⁢                                  ⁢                  Y          track                    =                                    t            disc                    2                ·                  Sin          ⁡                      (            θ            )                                ,       ΔYhead=tslider·Sin(θ), and
      θ    ≈                                        3            ·            Δ                    ⁢                                          ⁢          Z                          2          ·                      L                          track_to              ⁢              _ID                                          ⁢      for      ⁢                          ⁢              (                  Δ          ⁢                                          ⁢                      Z            head                    ⁢                      <<                          L                              track_to                ⁢                _ID                                                    )                                which        ⁢                                  ⁢        leads        ⁢                                  ⁢        to            ⁢                          ⇒      VC        =                  3        ·                  (                                    t              disc                        +                          2              ·                              t                slider                                              )                            4        ·        L            
Problems associated with disc flutter become more intolerable with higher track densities and disc rotation speeds.
Techniques have been previously developed by the assignee of the present application for compensating for track misregistration caused by disc flutter. Those techniques involved structures for the hinges coupling the load beam to the suspension arm, that introduced a vertical coupling as the hinges flex. That is, as the hinges flexed, a large vertical upward bending of the load beam caused a small horizontal movement at the slider end of the load beam.
In one embodiment, the previously developed approach involved introducing a vertical offset in the hinges that join the load beam to the suspension arm, with the vertical offset being created by various possible techniques including: attaching one hinge to the top of the load beam and the suspension arm, and a second hinge to the bottom of the load beam and the suspension arm; introducing a shim spacer between one of the hinges and the load beam, or between one of the hinges and the suspension arm, or both; and etching the load beam and/or the suspension arm to form either a lowered region or a raised mesa on the load beam and/or the hinge to which the suspension spring is mounted. The vertical coupling allowed the slider to track the designed data track on the disc platter as the platter bent due to vibration. As the platter bent upward, the load beam bent upward and the slider moved slightly horizontally toward the inside of the platter; conversely, as the platter bent downward, the load beam bent downward and the slider moved slightly horizontally toward the outside edge of the platter. In both cases, the result was that the slider stayed more closely aligned over the desired data track on the disc platter surface during disc bending. The foregoing techniques will be collectively referred to as offset suspension hinges as shorthand, although it will be understood that the techniques discussed below apply to the general case of compensating for vertical coupling within the load beam regardless of whether the vertical coupling within the load beam is created by offset hinges or by some other structure or arrangement.
If the suspension has a non-zero product moment inertia Iyz, then loading in the Z direction results in a spatial shift in the Y direction, i.e., the suspension has a YZ coupling. The movement is characterized by:
          =                            [          K          ]                ·                    =              c        ·                  [          I          ]                ·                                c      ·              [                                            Ixx                                      0                                      0                                                          0                                      Iyy                                      Iyz                                                          0                                      Iyz                                      Izz                                      ]            ·              [                                            0                                                          0                                                          Uz                                      ]              =          c      ⁡              [                                            0                                                                          Iyz                ·                Uz                                                                                        Izz                ·                Uz                                                    ]            Iyz=∫∫y·z·dA 
If symmetry in either the XZ plane or the YZ plane exists then:Iyz=0